212 



origin, and are exposed branching outgrowths, typically 

 from the outer sides of the visceral arches. 



Lungs are developed in many archaic Fishes, in Am- 

 phibians and in Reptiles, Birds and Mammals in which 

 the gill- clefts are vestigial and no longer respiratory. The 

 lung is a ventral outgrowth (dorsal in the highly specialised 

 Fishes) from the pharynx ; its connection with the pharynx 

 becomes the pneumatic duct or trachea (windpipe), and its 

 pharyngeal opening is the glottis. The highly vascular 

 lungs are hollow and thin-walled in Amphibians, and 

 become sponge-like organs in Birds and Mammals. In 

 the Dipnoi the lung is double in Lepidosiren and Protopterus ; 

 in Ceratodus it is single, and it also functions as a hydrostatic 

 organ. In certain Teleostean fishes the lung (a dorsal 

 outgrowth) remains connected with the pharynx (Physo- 

 stomi) ; in others (Physoclisti) it becomes shut off, a closed 

 sac, and mainly functions as a hydrostatic air or swim- 

 bladder, or float. 



Accessory air-sacs are developed from the lungs in Birds ; 

 these extend between the muscles and organs, and also 

 into the marrowless bones. 



The embryos of the higher Craniates breathe by the allan- 

 tois, an outgrowth from the hind-gut. (See pages 249, 260.) 



The skin is an important respiratory mechanism in 

 Amphibians (e.g., Frog). 



Describe the Excretory System of the Craniata. 



The nephrotome (see page 196) in the lower Craniates 

 (e.g., Elasmobranchs and Amphibia) remains connected 

 with the splanchnocoele by a narrow peritoneal canal. 

 Outgrowths from the nephrotomes (Malpighian capsules) 

 become the nephridia or kidney tubules. 



The inner end of a nephridium (opening either from the 

 nephrotome or the peritoneal canal) is the nephrostome. 

 A series of these nephridia, closely compacted, forms a 

 glandular renal organ or kidney (archinephros) ; and its 

 primary or archinephric duct is formed from the union of 

 the outer ends of the nephridia, and opens into the end 

 of the gut (proctodaeum). 



A plexus of capillaries, derived from the dorsal aorta, 

 forms the glomerulus, which bulges into the nephrotome. 

 From the circulating blood in the glomerulus, water is 

 extracted for flushing the nephridium and carrying away 

 the urea wastes extracted from the venous blood, which 

 .bathes the surface of the nephridium. 



